In general, an SOFC comprises a pair of electrodes (anode and cathode) that are separated by a ceramic, solid-phase electrolyte. To achieve adequate ionic conductivity in such a ceramic electrolyte, the SOFC operates at an elevated temperature, typically in the order of between about 700° C. and 1000° C. The material in typical SOFC electrolytes is a fully dense (i.e. non-porous) yttria-stabilized zirconia (YSZ) which is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Typical SOFC anodes are made from a porous nickel/zirconia cermet while typical cathodes are made from magnesium doped lanthanum manganate (LaMnO3), or a strontium doped lanthanum manganate (also known as lanthanum strontium manganate (LSM)). In operation, hydrogen or carbon monoxide (CO) in a fuel stream passing over the anode reacts with oxide ions conducted through the electrolyte to produce water and/or CO2 and electrons. The electrons pass from the anode to outside the fuel cell via an external circuit, through a load on the circuit, and back to the cathode where oxygen from an air stream receives the electrons and is converted into oxide ions which are injected into the electrolyte. The SOFC reactions that occur include:Anode reaction: H2+O═→H2O+2e−CO+O═→CO2+2e−CH4+4O═→2H2O+CO2+8e−Cathode reaction: O2+4e−→2O═
Known SOFC designs include planar and tubular fuel cells. Applicant's own PCT application Nos. PCT/CA01/00634 and PCT/CA03/00059 disclose methods of producing a tubular solid oxide fuel cell by electrophoretic deposition (EPD), metal electrodeposition (MED) and composite electrodeposition (CED). The fuel cell comprises multiple concentric layers, namely an inner electrode layer, a middle electrolyte layer, and an outer electrode layer. The inner and outer electrodes may suitably be the anode and cathode respectively, and in such case, fuel may be supplied to the anode by passing through the tube, and air may be supplied to the cathode by passing over the outer surface of the tube. The methods taught by these two applications are particularly useful for producing a small-diameter “micro” fuel cell that is suitable for powering small scale applications such as portable electronic devices.
Multiple fuel cells can be electrically and physically coupled together to form a stack to provide power to a load. In certain applications, the load can vary with time; various fuel cell systems have been proposed wherein the power supplied by the stack follows the varying load. When an SOFC stack output is following a varying load, there may be instances when the stack is not operating at an acceptable efficiency. For example, fuel utilization rates and fuel cell operating temperatures will change as the stack output changes, and may fall outside an acceptable operating range. It is therefore desirable to provide an operating strategy that enables a load-following fuel cell stack to operate in an efficient manner. Such operating strategy is particularly important where the fuel cell stack is used in a portable application where the fuel supply may be limited, and power management is an important consideration.
Also during operation, the fuel cells in the stack should be maintained within a particular temperature range in order to provide stable power output. Therefore, it is also desirable to provide balance of plant components for the stack and an operating strategy for the stack that maintains the stack within the desired operating temperate range, as well as within other desired operating parameters such as efficiency and fuel usage rates.